The present invention pertains to IV pumping apparatus, and particularly to volumetric pumps suitable also for controlling passive infusion and a method for achieving said control.
To improve health care, there has been considerable effort with regard to the administration of intravenous (IV) fluids. For many years IV solutions were adminsistered only by the force of gravity. Generally, the volume rate was measured by counting the number of drops per minute. This method, however, has proved unsatisfactory for several reasons. For example, the drop size can vary considerably, since drop size is directly proportional to surface tension, which is affected by temperature and type of solution. Also the drop size is affected by the speed at which the drop forms. Further, the drop rate is affected by the restrictions of the tube and needle, and the drop-forming geometery. If a tube is partly occluded, the drop rate will decrease or as the IV supply decreases the hydrostatic pressure will decrease causing a decrease in drop rate. In many cases, therefore, the variability of both the drop size and the drop rate (both of which are for the most part beyond the control of the operator) makes this method of administration of intravenous fluid unsatisfactory.
Improvements have been made by adding an electronic drop counter together with either a controller or a peristaltic pump. The electronic drop counter and controller combination controls the drop rate, but makes no improvements in controlling drop size, and also has the deficiency of not being able to control drop rate if back pressure increases beyond the hydrostatic forcing pressure. The electronic drop counter and peristaltic pump combination increases the forcing pressure but lacks an accurate metering method.
Improvement in metering methods results with the use of displacement pumps, which offer the capability of greater precision in controlling IV flow rates than is possible with the prior art IV controllers which depend on gravity. These pumps, in addition to metering the fluid, also apply positive pressure to the fluid or the IV tubing. The displacement pump can generally be categorized as peristaltic (such as described in U.S. Pat. No. 3,737,251 by Berman et al) or piston-cylinder (such as described in U.S. Pat. No. 3,985,133 by Jenkins et al) or pulsating (such as described in U.S. Pat. No. 3,874,826 by Lundquist).
The peristaltic pump, although generally an improvement over the prior art, has a number of disadvantages. First, stretching an elastomeric material in a peristaltic manner does not lend itself to an efficient use of energy. Second, repeated stretching can irreversibly deform the material, causing changes in pump displacement, and possible failure. Further, because patients can be ambulatory and because of potential loss of power, a battery backup for the pump is required. Hence, it is desireable that the pump be as efficient as possible.
The piston-cylinder pumps of the prior art provide for accurate metering and positive pressure, but also have several disadvantages. First, because intravenous therapy requires that the pump maintain a sterile barrier, and cost prohibits cleaning and sterilization after each use, the pumping chamber must be disposable and inexpensive to manufacture. This has been difficult to achieve with the prior art piston-cylinder pumps. To reduce manufacturing costs, some prior art pumps use only one chamber and two valves. This requires that the pump cycle have two parts, a fill, and an empty; therefore, IV therapy is interrupted during the fill portion of the cycle. Second, the friction of a piston-cylinder pump is a cause for reduced efficiency.
The pulsating pumps provide a continuous pulsing flow, but also have significant disadvantages. First, the self-contained valving of these pumps has added to the complexity and expense of the disposable pump chamber. Second, the pulsing action against a spring load or an elastomeric material does not lend itself to efficient operation.
U.S. Pat. No. 3,809,507 by Baglai describes a pump which is not intended specifically for use in IV therapy, but which does provide a continuous pulse-free flow. The valves in this pump are either located on moving parts or located at a fixed location and connected by a flexible tube. This approach does not lend itself to an economical disposable pump chamber as is required for IV applications. Also, without the valves biased or powered, the pump in the off condition may continue to supply fluid (i.e. it suffers from "siphoning"). This is an unsafe condition for IV therapy.
Another pump design which is specifically adapted for infusing IV fluids is described in U.S. Pat. No. 4,410,322 by Archibald. That system also provides for a continuous flow and has a disposable pumping chamber. In that system, however, the individual pumping chambers are of a rolling diaphragm type design similar to the rolling diaphragm system described in U.S. Pat. No. 2,849,026 issued to Bellofram Corporation. A significant problem associated with such rolling diaphragm systems, however, is that the chamber wall has a tendency to collapse under negative gauge pressures, and renders the pumping chamber inoperative when pressure is restored. Hence, specific measures are taken to avoid negative pressure differentials across the diaphragm wall. Such negative pressures are often encountered in the patient care environment when a patent's infusion site is below the level of the pump.
What is needed is an energy efficient pump with an inexpensive disposable flow chamber that can deliver accurate volumes of infusate over a wide range of system pressures of clinical concern. Furthermore, it would be desirable if the pump itself could also be used as a flow controller in order that active pumping not be required continuously in order to deliver precise volumes of infusate and in order that the pump not be required to be removed for passive infusion.